Watch designers are constantly seeking to use hands with increasingly higher unbalances in watches. Indeed, design often requires long hands, and/or hands fabricated from more noble materials than aluminium, or materials better suited to a heat and/or surface treatment. In particular, the use of brass permits electroplating with an attractive appearance. The use of precious metals and alloys, gold, platinum or suchlike, enables high end watches to have hands enjoying the same standard as the appliques and case middle.
Excessive unbalance is not advantageous, particularly in the event of a shock. An electronic watch may thus experience a motor step loss in the event of a shock, caused by the unbalance of the hand.
It is known to use a counterweight directly on the hand to reduce the unbalance of the hand, by moving the centre of gravity of the assembly towards the centre of rotation. However, this counterweight is generally directly incorporated in the hand and affects its aesthetic appearance.
A better solution consists in combining a hand with a mobile element acting as a flywheel and having an opposite unbalance to that of the hand, so that the resulting unbalance is as low as possible when the mobile element and hand are attached to each other. However, the mobile element is then located underneath the dial, and direct optical orientation adjustment is thus impossible, since the hands are pressed in after the dial is mounted, and the gear trains are in most cases no longer visible, which makes any optical or visual adjustment impossible.
Patent document JP5299667 in the name of CASIO proposes a solution to this problem of alignment between a hand and a mobile element comprising a counterbalance, and proposes the use of a light source, arranged to illuminate one area of the mobile element comprising a small positioning bore, and a light sensor on the opposite side to the mobile element. Where several hands are used, each pressed onto one such mobile element, the bores must be aligned for common indexing. Although this solution can be envisaged during an initial assembly operation, it is not suitable for maintenance during service, when, for example, the hands need to be removed then replaced in order to work on the movement, or to perform more precise angular setting.
JP Patent Application S55 55275A in the name of CITIZEN discloses a similar optical system using the reflection of a light ray on a mirror.
JP Utility Model S52 109851U in the name of TOKO discloses an inertia block system opposite to a hand.
It is therefore necessary to develop another method of alignment between the hands and such a mobile element.